1. Field of the Invention
This invention relates to removal of nitrogen oxides and a catalyst used therefore. Particularly it relates to a catalyst the activity of which is not reduced easily due to volatile metal compounds contained in exhaust gases and a process for removing nitrogen oxides by the use of the catalyst with a good efficiency.
2. Description of the Related Art
The process of reducing and removing nitrogen oxides (NO.sub.x) contained in various exhaust gases with ammonia (NH).sub.3 is simple in the system and efficient; hence the process has constituted a main stream of denitration processes for exhaust gases generated from various sources, including boiler combustion exhaust gases. For the process, the so-called denitration catalysts for promoting the reaction of NO.sub.x with HN.sub.3 have been necessary and a number of inventions have so far been made. Among the catalysts, those which have currently been practically used are represented by catalysts disclosed in Japanese patent application laid-open Nos. Sho 50-51966/1975 and Sho 52-122293/1977, that is, those obtained by adding oxide(s) such as vanadium (V), molybdenum (Mo), tungsten (W), iron (Fe), etc. to titanium oxide (TiO.sub.2) as a main component. These catalysts are superior in that even when ionic oxides are contained in exhaust gases, they are difficultly deteriorated to exhibit a high denitration performance.
However, as to such conventional catalysts, no consideration has been taken about reduction in the catalyst activity caused by volatile metal compounds formed mainly from mineral components contained in fuels, i.e. oxides of selenium, tellurium, thallium, arsenic, etc. and compounds of alkali metals, alkaline earth metals, etc. together with the above compounds. In recent years, coal and oil containing a large quantity of mineral substances have been used to bring about a tendency that the concentration of the above volatile metal compounds contained in exhaust gases increases. However, when the above-mentioned conventional catalysts are used for denitration of exhaust gases, a problem has been raised that the activity is lowered to a large extent.